1. Field of the Invention
The present invention relates to a 1-axis type combined cycle plant. Especially, the present invention relates to a 1-axis type combined cycle plant in which the output efficiency is improved.
2. Description of the Related Arts
As a high efficiency power generation system, a combined cycle plant is known. In the combined cycle plant, steam is generated by using hot exhausted gas discharged from a gas turbine, and a steam turbine is driven with the steam. A generator is driven by the gas turbine and the steam turbine. By using the thermal energy of the exhausted gas from the gas turbine, the combined cycle plant can achieve high efficiency. As a combined cycle plant, a 1-axis type combined cycle plant is known. In the 1-axis type combined cycle plant, a turbine axis of gas turbine, a turbine axis of steam turbine and a rotor of generator are coaxially connected and combined as a unit. Generally, a power plant includes a plurality of sets of the gas turbines, the steam turbines and the generators. The 1-axis type combined cycle plant has advantages that the efficiency on partial load is high and the starting time is short, resulting in superior operativity. An example of a method of controlling such a 1-axis type combined cycle plant is shown in Japanese Laid Open Patent Application (JP-A-Heisei 11-332110).
In order to increase the output of the combined cycle plant, a duct burner is sometimes used. The duct burner sprays fuel into the exhausted gas discharged from the gas turbine and burns the fuel to raise the temperature of the exhausted gas. The rising of the temperature leads the increase of steam and thereby the increase of the output of the steam turbine. As a result, the output of the combined cycle increases.
In the 1-axis type combined cycle plant, the use of a duct burner has great influence on the combustor of the gas turbine. In the case of 1-axis type combined cycle plant, the output of the steam turbine varies in response to a flow rate of fuel supplied to the duct burner and therefore the output of the generator varies. As the gas turbine is controlled in response to the output of the connected generator, the flow rate of the fuel supplied to the duct burner influences on the gas turbine directly. Especially, rapid decrease of the flow rate of the fuel supplied to the duct burner affects crucial influence on the gas turbine. If the flow rate of the fuel supplied to the duct burner decreases rapidly, the output of the generator decreases rapidly. In this case, the flow rate of the fuel supplied to the gas turbine increases rapidly to compensate the rapid decrease of the output of the generator. The rapid increase of the flow rate of the fuel supplied to the gas turbine often causes damage to the combustor of the gas turbine. When the duct burner is urgently stopped due to a defect while a lot of fuel is supplied to the duct burner, the influence on the gas turbine is more serious.
A 1-axis type combined cycle plant with a duct burner is demanded to have a function to protect the combustor from the damage caused due to rapid decrease of the flow rate of the fuel supplied to the duct burner.
In conjunction with the above description, a control apparatus of a power generation plant is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 11-324726). In this reference, a speed control load control signal is determined based on a power generation instruction value which is instructed based on change of a system frequency, measured values of generated power and axis speed, and a predetermined control rate. In the power generation plant with a turbine and a generator which are connected with each other via an axis, load control of the turbine is carried out based on the speed control load control signal to stabilize the system frequency. At this time, the load control of the turbine is carried out based on an operation instruction value which is determined by multiplying the speed control load control signal by a load limitation function to limit the change of the speed control load control signal.